In water-cooled car engines, the cooling system may for example comprise a water jacket fitted to an engine cylinder block and cylinder head, a radiator and a water pump. The water pump is driven by an engine crankshaft via a belt, cooling water being recirculated between the water jacket and radiator.
Water which has been heated by absorbing engine heat in the water jacket is led to the radiator, where this heat is dissipated to the atmosphere.
The radiator is cooled by a radiator fan and by wind depending on the vehicle speed. Modern cooling devices are provided with a fan rotated by an electric motor so that a predetermined wind volume is supplied when the vehicle speed and engine speed are low, such as when the vehicle is climbing a hill or in a traffic jam.
In the same way as a fuel injection amount from a fuel injector is centrally controlled by an ECU (Electronic Control Unit), the radiator fan is controlled by a command signal from the ECU, and started and stopped according to vehicle speed, cooling water temperature and air conditioner operating conditions.
However, if the radiator fan breaks down when the vehicle is climbing a hill or in summer so that it is no longer able to dissipate heat, cooling water reaches boiling point and steam is generated which escapes from the overflow pipe.
To prevent this overheating, a correction may be made to increase the amount of fuel injected by the injector when the cooling water temperature exceeds for example 100.degree. C. Alternatively, Tokkai Hei 4-143438 published by the Japanese Patent Office in 1992 discloses a method wherein fuel is cut off regardless of running conditions when the cooling water temperature rises above for example 145.degree. C.
However, according to the aforesaid former method where the fuel amount is increased for high water temperature, the air-fuel ratio tends to be richer than the theoretical air-fuel ratio which leads to an increase of CO and HC in the exhaust. According to the latter method, if fuel is cut off when it is desired to accelerate, the driver cannot obtain the desired acceleration.